The operational functions of artificial Earth satellites usually require the downlink of data to ground stations. Examples of downlinked data include engineering telemetry, sensor readings, scientific measurements, or relayed communications information passed from an uplink signal. Radio transmission is a ubiquitous method for satellite downlink in the present art. A few satellites have supplementary lasers, generating a narrow beam of collimated light, for additional high-bandwidth downlink capacity to a ground station within the laser beam.
The operation of radio transmitters in space is highly regulated, primarily due to the risk of interference with other signals. Satellites operate at an extremely high altitude compared to terrestrial radio sources, and a large portion of the Earth's surface is in direct line-of-sight from a satellite at any time. Satellites in near-Earth orbit move rapidly around the globe, and pass directly over numerous nations and geographic regions. A malfunctioning radio transmitter in space has the potential of causing interference to terrestrial radio communications anywhere on the planet. Control of a malfunctioning radio transmitter in space can be impossible, since it is physically inaccessible to maintenance personnel. For this reason, regulations governing the use of radio transmitters in space require the availability of an uplink channel, so that a ground station can remotely command the shutdown of the satellite's transmitter. This safety precaution, however, cannot protect against a failure in a satellite's control system that effectively severs control of the radio transmitter, or a failure in a satellite's uplink receiver that interferes with the satellite's ability to receive ground commands.
For several reasons, compliance with regulations governing operation of a radio transmitter in space can be burdensome to the development of small ‘microsatellites’ or ‘nanosatellites’, extremely small spacecraft constructed using minimal mass and complexity. Small satellites are beneficial due to the ability to ‘piggyback’ on surplus launch capacity, the potential use in ‘swarms’ where numerous low-cost spacecraft collectively provide regional coverage or undertake high-risk activities, and educational value when spacecraft design or construction occurs in the context of an academic project.
One compliance burden is the requirement for inclusion of a potentially unnecessary subsystem on the spacecraft, an uplink communications receiver. Some satellites may have reason to return data to Earth, but have no inherent need for an uplink channel, other than providing the capacity for remote shutdown of the downlink transmitter. Examples include evaluation of thermal or attitude performance of spacecraft prototype configurations, long-duration testing of the performance and reliability of components that have not been qualified for use in critical spacecraft functions, and measurement of transient events such as space debris or micrometeroid impacts.
Another compliance burden is working through the process for obtaining permission to broadcast from space on an assigned radio frequency. This may introduce significant expense or delay, and is problematic for spacecraft that must be developed on a very short time scale. For example, an unexpected launch opportunity may be generated when a manifested secondary payload is removed from a rocket for technical reasons, or when evaluation of primary payload mass reveals unused mass to orbit capacity that may be utilized by the addition of a small secondary payload. Developers of very small spacecraft cannot purchase dedicated launch vehicles, and rely on the ability to respond promptly to such unanticipated circumstances. In this context, risk is introduced by the necessity of certification for a transmitter shutdown system and regulatory authorization to use a specific downlink radio frequency.
Various methods are known for short-range optical communications using uncollimated light. For example, optocouplers use uncollimated light to transfer information within an electrical component or circuit board, and television remote control devices use uncollimated infrared optical signals to communicate over a few meters. When devices can be physically connected, fiberoptics can be used to extend the communications range. Optical communications systems requiring greater performance are typically implemented with lasers. U.S. Pat. No. 5,726,786 to Heflinger (1998) describes the implementation of an uncollimated optical data bus for connecting subsystems within a spacecraft, or for communications between a spacecraft and other equipment virtually in contact with the spacecraft (e.g. to ground support equipment on the launch tower before liftoff, or to another docking spacecraft in orbit). The effective range of an optical data bus is measured on the order of centimeters or a few meters, evidently not useful for communications between Earth and an orbiting satellite, where the ranges are at least hundreds of kilometers. Further, an optical data bus captures and retransmits optical signals to interconnect a plurality of optical transceivers with bidirectional communications paths between any pair of transceivers, and does not provide for the outbound broadcast of information from a single data source, without optical relay, to optical receive-only stations with no symmetric capacity for participation in the optical data bus, and possibly in a plurality of locations unknown to the optical transmission source.
For optical communications over longer ranges, such as between Earth and a satellite, laser optical downlink systems have been developed as an alternative to radio. However, laser transmission from a satellite requires extremely precise control of the spacecraft's orientation, so that the narrow laser beam is delivered to a ground station facility. The directionality of a laser beam is a primary motivation for the development of laser satellite communications in the prior art, and is specifically cited as an advantage of such methods. As explained in U.S. Pat. No. 6,097,522 to Maerki, et al (2000): “Thanks to the extremely short wavelength of light, an optical beam can be radiated very easily by means of a relatively small optical device at a narrow space angle. By means of the antenna gain achieved in this way, a high data rate can be transmitted with low transmission output. Corresponding directional antennas for microwave connections are comparatively heavy and require a relatively large space. However, because an optical transmitted beam can be easily collimated, it requires an extremely exact determination and tracking of the direction of the transmitted beam as well as that of the reception direction.”
According to U.S. Pat. No. 6,922,430 to Biswas, et al (2005), “In order for a satellite to optically communicate with a ground station, it must be able to orient its antenna, which is often highly directional, toward the ground station. The ground station must therefore send up a directional beacon to the satellite on which the satellite can lock for orientation purposes.” The pointing accuracy required for laser communications is extraordinary. U.S. Pat. No. 5,475,520 to Wissinger (1995) notes that “Optical beams of sufficient brightness are typically tens of microradians in diameter, while the corresponding requirement for RF beamwidths is generally on the order of one to two degrees. Acquisition and tracking of the beam is problematic in that the beam must be pointed at a remote transceiver with microradian accuracy.”
To put this in perspective, the angular diameter of a typical downlink laser beam is smaller than the angular diameter of the planet Neptune in Earth's sky. Aiming a satellite with such precision is, indeed, “problematic”. The task is so demanding that optical downlink, with the present art, is simply out of reach for nanosatellites. Further, the narrow size of a laser beam makes such a system unacceptable as a satellite's primary downlink system, since the process of co-ordinating a laser downlink session may require two-way communications with the spacecraft, and reliance on an extremely directional downlink would make recovery practically impossible from a spacecraft anomaly or “safe mode” condition.
In addition, the narrow illumination footprint of a laser downlink signal on the Earth's surface, typically smaller than 1,000 feet in diameter, makes reception contingent on favorable weather conditions at the observing site. This is a known disadvantage to laser downlink systems, and satellite detection from high-altitude balloons have been proposed to overcome this limitation, as in U.S. Pat. No. 7,046,934 to Badesha, et al (2006).
Therefore, although laser downlink methods are not subject to certain regulatory and safety complications associated with the use of a radio transmitter in space, the sophistication of laser instruments and the requirement for extremely precise spacecraft orientation make lasers inappropriate for applications with very small spacecraft. Further, a laser system cannot replace the functionality of a less directional radio downlink system, so the use of lasers cannot obviate the regulatory issues connected to authorization and use of space-based radio transmissions.
Accordingly, there is a need for a satellite downlink method providing a broad beam or non-directional signal, without requiring frequency assignment and coordination, and without posing a risk to terrestrial activity in the event of a satellite malfunction. Further, there is a need for an optical satellite downlink method that is functional without precise aiming of the spacecraft towards a ground station. In addition, there is a need for a satellite downlink method that does not require the inclusion of an attitude control system or an uplink radio receiver on the spacecraft, or if such subsystems are present, can safely provide downlink communications in the event of the failure affecting these subsystems.